 First, during the early Vienna years, when Mozart was influenced by Baron van Swieten to take up the study of Baroque counterpoint, Haydn lent him his personal copy of the famous counterpoint textbook "Gradus ad Parnassum", by Johann Joseph Fux, which was heavily covered with Haydn's personal annotations.In addition, like many other younger musicians, Mozart addressed Haydn with the honorific term "Papa".For details of this form of address, see Papa Haydn.